1. Field of the Invention
The invention relates to a telecommunications system comprising at least one subscriber station and a circuit arrangement connecting a subscriber station by means of a reversing switch either to an AC signalling voltage generator or a line voltage supply.
2. Description of the Related Art
A telecommunications system of this type is known in the art for use, for example, as telephone exchange. The publication entitled "Telecommunications Protection Circuits, Protection Design Guide" Texas Instruments, 1986, DL 32-11/86, S/C Rev. 6, discusses several circuit arrangements in which either an AC voltage generator or a subscriber station DC line voltage supply circuit can be connected to at least one subscriber station by means of a reversing switch. While the AC voltage generator is so connected a further DC voltage generator (battery) delivers a DC voltage superimposed on the AC voltage to the subscriber station. When there is a call, the reversing switch connects the line voltage supply circuit to the subscriber station so as to deliver, for example, a DC operating voltage.
Generally, a plurality of subscribers' stations are connected to the reversing switch by means of an automatic change-over switch (AWADO). Such an automatic change-over switch needs a DC voltage to operate faultlessly. For that purpose, a DC voltage is superimposed on the AC voltage. In order to minimize the circuitry, coilless AC signalling voltage generators without a DC voltage generator are preferable. In that case, however, the automatic change-over switch cannot operate faultlessly.